Background of Our Research: DNA damage can be a result of both cell-intrinsic sources (such as reactive oxygen species) and a variety of extrinsic factors, including UV light, ionizing radiation (IR) and radiomimetic chemicals. Consequently, if DNA damage is left unrepaired, it may cause cell death and genomic instability, which can lead to mutation and chromosome rearrangement, and predisposition to cancer. Cell death by DNA damage may cause early aging as well. To understand the mechanisms of DNA damage response is important for the prevention of cancer and cancer therapy.